1. Field of the Invention
The present invention relates to a toner useful for developing an electrostatic latent image formed on an image bearing member, and to an electrophotographic image forming method in which an electrostatic latent image formed on an image bearing member is developed using a toner to form a visual image.
2. Discussion of the Background
Inorganic photoconductive materials such as selenium, zinc oxide, cadmium sulfide and selenium alloys have been developed and used for an electrophotographic photoconductor. In recent year, organic photoconductive materials (OPC) are mainly used for a photoconductor because the materials have the following advantages:
(1) having a relatively low cost; PA1 (2) being easily processed; PA1 (3) capable of producing images having good image qualities; and PA1 (4) being environmentally friendly even when disposed of.
However, the disadvantage of the OPC photoconductors against inorganic photoconductors is that the properties of the photoconductors are easily influenced by environmental changes in image forming apparatus because the charge transporting layer (CTL) of the OPC photoconductors is mainly constituted of an organic polymer. One of the environmental changes is generation of acidic gases such as ozone and NOx, which is caused by a main charger for charging the photoconductor, and a transfer charger. When OPC photoconductors are exposed to such acidic gases, decomposition of a charge transporting material (CTM) included in the CTL occurs at the surface of the CTL. Therefore the resultant images tend to be blurred. In addition, charge quantities of the OPC photoconductor are often decreased by the acidic gases depending on the charge generating material (CGM) used in the charge generating layer (CGL), thereby causing background fouling and deterioration of image density of the resultant mages. Further, there occurs a contamination problem in that the toner, which remains on a surface of an OPC photoconductor even after the photoconductor is cleaned, contaminates the OPC. Namely, when an OPC photoconductor contacts the toner, which is electrostatically adhered on the developing sleeve or the toner remaining on the surface of the photoconductor, for a long time under a high temperature condition, the toner contaminates the OPC photoconductor, resulting in increase of the residual potential of the OPC photoconductor, and thereby blurring and tailing occur in the resultant images. Since the copying speed of electrophotographic image forming apparatus now increases more and more, it is very important to solve these problems.
In addition, Japanese Patent Publication No. 8-27552 discloses a toner including a composition of a styrene copolymer (B) which includes a styrene-including oligomer (A) having a number average molecular weight not greater than 1000 in an amount of from 0.01 to 1% by weight. Namely, the toner includes the styrene-including oligomer in an amount of from 100 ppm to 10000 ppm. However, when processes, in which an OPC photoconductor is developed with such a toner and the resultant toner images are transferred, are repeated for a long time, the OPC photoconductor is contaminated with the toner. In particular, the residual potential of the OPC photoconductor increases seriously, resulting in occurrence of background fouling and blurring in the resultant images. The reason is considered to be that the oligomer migrates into the OPC photoconductor and traps charges, resulting in prevention of transporting of the charges, and thereby the resultant image has image defects.
Because of these reasons, a need exists for an electrophotographic toner for developing latent images formed on an OPC photoconductor, which produces images without image defects caused by contamination of the OPC photoconductor even when the toner is used for a long time even under a relatively high temperature condition.